1. Field of the Invention
The present invention relates to a variable capacity hydraulic pump for a device utilizing pressure fluid, such as a power steering device, which reduces a steering power in a vehicle, for example.
2. Description of the Related Art
A capacity type vane pump directly driven by a vehicle engine has been used for a pump for use with a power steering device of this type. In the capacity pump of this type, quantity of a working fluid varies in dependence upon a engine speed. Therefore, an auxiliary steering power is increased when the vehicle stops or runs at low speed, and it is decreased when the vehicle runs at high speed. That is, the capacity pump is required to have a characteristic contradictory to a characteristic on an auxiliary steering power required for the power steering device. For this reason, it is necessary to use a capacity pump of a large capacity which is capable of producing such a quantity of the discharge fluid as to secure a necessary auxiliary steering power even when the vehicle runs at low speed. Additionally, it is essential to use a flow control valve which controls the quantity of the discharge fluid to be a predetermined value or smaller. As a result, the number of required component parts is relatively large, the structure and passage arrangement are complicated, and consequently it is unavoidable that the overall pump structure is increased in size and cost.
To solve the problems, there is proposed a variable capacity vane pump capable of decreasing the quantity of the discharge quantity per revolution (cc/rev) of the pump in proportion to the number of revolutions, as disclosed in JP-A-6-200883, 7-24338 and 8-200239. In each of those pumps, there is eliminated the use of the control flow valve attached to the capacity pump. As a result, the drive horsepower is reduced. In this respect, the proposed pump is excellent also in energy efficiency.
In the pump disclosed in JP-A-8-200239, for example, as shown in FIG. 7, a rotor 3 is disposed eccentrically relative to and within a cam ring 2, which is swingably supported within an elliptical space in a pump body 1, whereby a lunette pump chamber 4 is formed between the rotor 3 and the cam ring 2. First and second pressure chambers 5 and 6 are formed on both sides of the cam ring 2 as viewed in the swing direction. A compression spring 2b, which urges the cam ring in such a direction as to maximize the pump capacity of the lunette pump chamber 4, is provided in the second pressure chamber 6. A control valve 10 is provided which is operated by a fluid pressure difference between the upstream and downstream sides of a variable metering throttle 12 provided at the mid position of a discharge path 7 through which a pressure fluid discharged from the lunette pump chamber 4 flows. The cam ring 2 is swung through the control of the fluid pressures in the first and second pressure chambers 5 and 6 by the control valve 10.
A hole 12a is bored in the side wall of the pump body 1, which faces the inside of the second pressure chamber 6. An opening area of the hole 12a is adjusted with an outer peripheral edge 12b of the cam ring 2 when the cam ring 2 is swung, whereby the variable metering throttle 12 is formed. That is, the pump has a structure to directly introduce the fluid pressure of a downstream side of the variable metering throttle 12 into the second pressure chamber 6 of those chambers 5 and 6 for swinging the cam ring 2. The pressure fluid discharged from the lunette pump chamber 4 is introduced into the hole 12a defining the variable metering throttle 12. The pressure fluid flowing into the second pressure chamber 6 through the hole 12a is sent to a pump discharge path 13 via the second pressure chamber 6, and discharged through an outlet port, not shown.
The variable capacity hydraulic pump employs such a structure that when the pump operates in high speed region, a fluid pressure in the upstream side of the variable metering throttle 12 is introduced, by means of the control valve 10, into the first pressure chamber 5 through a passage 5a having a damper throttling function. In such a structure, when the cam ring 2 swings toward the first pressure chamber 5, a given braking force is caused to act on the cam ring 2 by the damping function of the passage 5a. 
As described above, the compression spring 2b is merely provided for the second pressure chamber 6, and the second pressure chamber 6, unlike the first pressure chamber 5, is not provided with means having a damping function for applying a braking force to the cam ring 2. The reason for this is that the hole 12a of the variable metering throttle 12 is opened to the second pressure chamber 6, and are used also as parts of the discharge paths 7 and 13. The fluid pressure in the discharge side of the second pressure chamber 6 propagates through those paths. In the figure, reference numeral 6a designates a passage for leading the fluid pressure in the downstream side of the variable metering throttle 12 to the other chamber of the control valve 10.
Accordingly, when the cam ring 2 swings toward the second pressure chamber 6, a resilient force that is generated by the compression spring 2b when it is flexed acts on the cam ring, but it is impossible to apply a damper basis braking force to it. Therefore, the swing of the cam ring 2 to the first and second pressure chambers 5 and 6 (particularly the swing of it from the first pressure chamber 5 to the second pressure chamber 6) is not smooth. If the swing of the cam ring is not smooth, it is unavoidable that the cam ring 2 vibrates, and a pulsation occurs in the fluid pressure in the pump discharge side. A pulsating variation of the fluid pressure is shown as a waveform indicated by a broken line in FIG. 8.
This will be described in more detail. The fluid pressure of the pump discharge side flows in a jet stream from the hole 12a that is opened to the second pressure chamber 6, into the second pressure chamber. When the opening of the hole 12a is opened or closed with the outer peripheral edge 12b of the cam ring, the cam ring 2 tends to vibrate. Further, when the jet stream of the pressure fluid flowing out from the hole 12a is inhibited or permitted to flow, the pulsation of the fluid pressure increases. When the vibration and pulsation occur in the fluid, the power steering device suffers from variation of the steering force, growing of noise, e.g., fluid sound, and the like.
An object of the present invention is to provide a variable capacity hydraulic pump which suppresses the vibration of the cam ring and the pulsation of the fluid pressure of the pump discharge side.
To achieve the object, the present invention provides a variable capacity hydraulic pump in which a cam ring forming a pump chamber is swingably supported within an inner space of a pump body, a first fluid pressure chamber is formed on one side of the cam ring as viewed in the swing direction, a second fluid pressure chamber is formed on the other side of the cam ring, urging means is provided for urging the cam ring in such a direction as to maximize a capacity of the pump chamber, a control valve is provided which operates in response to a fluid pressure difference between the upstream and downstream sides of a variable metering throttle provided in the middle of a discharge path through which pressure fluid discharged from the pump chamber flows, and a fluid pressure within at least the first fluid pressure chamber is controlled by the control valve to swing the cam ring. The variable capacity hydraulic pump is improved in that the variable metering throttle is provided at a position which is located between the side surface of one side of the cam ring as viewed in the axial direction and the side wall of the pump body which faces the side surface, and is structurally isolated from the second fluid pressure chamber.
In the thus constructed variable capacity hydraulic pump, the variable metering throttle is provided at a structurally isolated position which does not affect a fluid pressure within the second fluid pressure chamber. Therefore, the cam ring may be swung while be given damping functions by the fluid pressures in the first and second first fluid pressure chamber.
The invention also provides another variable capacity hydraulic pump which is constructed as described above, and additionally has the following construction: the variable metering throttle is formed with a communicating channel which is formed in the side surface of one side of the cam ring in the axial direction, and communicates with a discharge-side hollow being formed in the side wall of the pump body which faces the side surface, and a small hole which is bored in the side wall of the pump body at a position facing a part of the communicating channel, and of which the opening area is varied with the side edge of the communicating channel of the cam ring when the cam ring swings.
In the last-mentioned invention, a degree of throttling of the variable metering throttle may be adjusted in a manner that the small hole of the side wall of the pump body, which faces the communicating channel formed in the side surface of one side of the cam ring, is closed or opened with the side edge of the communicating channel of the cam ring when the cam ring swings.